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The invention relates generally to optical spectrometers, and more specifically to an optical spectrometer using a variable etalon optical filter structure in conjunction with an optical detector array.
Optical telecommunications systems often carry several optical channels on a single optical fiber using a technique known as wavelength-division multiplexing (xe2x80x9cWDMxe2x80x9d). The channels generally carry data such as voice transmissions, pictures, and/or video as a digital signal, but analog signals might also be carried in some instances. Channels are often described according to their nominal center-channel wavelength. The spacing between channels has continued to decrease as demand for optical communications has grown. Decreasing channel spacing allows more channels to be sent over an existing optical fiber, thus increasing capacity without laying additional optical cable. In current dense WDM (xe2x80x9cDWDMxe2x80x9d) systems, the channels may be spaced only 100 GHz (0.8 nm at 1550 nm) or 50 GHz apart (0.4 nm at 1550 nm), with even closer channel spacing being desirable.
As the demand for higher performance optical telecommunications systems grows, so does the need to characterize the components and systems. Performance of optical components, systems, and subsystems can be defined and measured in many ways, such as by signal strength, signal-to-noise ratio, and signal wavelength, including wavelength stability. The measurements might be made as an initial measurement when the component or system is first built, or monitoring of a system or signal path might be done on a continuous or periodic basis. Measuring the wavelength(s) of an optical signal is often done with an optical spectrometer.
Generally speaking, an optical spectrometer provides an indication of amplitude versus wavelength for an optical input. An optical signal analyzer, which often uses a movable grating or filter in association with an optical detector or detectors, provides a similar function. Many conventional optical spectrometers use dispersive elements, such as a diffraction grating or prism, to spread the optical signal into its constituent wavelengths with a detector array that has detectors positioned to measure the signal strength of the wavelength associated with that position. These techniques require a relatively large device to accommodate the rate of dispersion of the signal, and may be susceptible to shock and vibration moving the dispersive element with respect to the detectors. Such misalignment could result in measurement errors and require frequent calibration and/or alignment.
Another approach to optical spectroscopy utilizes a rotating variable bandpass filter in conjunction with a wide band detector. The rotating filter is placed between the detector and the light source. The light entering the detector from the light source depends on which portion of the filter has been rotated between the source and the detector. The filter is typically rotated with a stepper motor, thus achieving a fairly accurate and repeatable position that allows for calibration of the system. One technique uses a filter with relatively few layers and a relatively wide bandpass characteristic so that only about half of the incident light is transmitted through the filter at each position of the filter, thus the same wavelength light is detected at multiple filter positions. This improves the measured signal intensity, but resolution suffers. The resolution can be enhanced with a thorough characterization of the system and calibration, but this approach is generally suited more for lower resolution spectroscopy, such as chemical analysis and in-vivo blood testing, such as a blood glucose monitor, than for applications requiring high resolution of closely spaced optical signals. This approach also relies on the mechanical movement of the filter, which increases the complexity and opportunity for mechanical failure of the system.
Another approach avoids the need for a mechanically moving filter by combining a variable filter with a detector array. Each detector (pixel) in the detector array is exposed to a different portion of the spectrum according to its position relative to the variable filter. The filter could be a variable long-pass, short-pass, or band-pass filter. The spectral resolution may be adjusted, within limits, by selection of the filter""s spectral spread and number and spacing of detectors. Each pixel in the detector array is covered by a different portion of the filter and thus exposed to a different spectral transmission through the filter. However, conventional spectrometers using this type of filter-detector have limited resolution due to limitations in detector and filter fabrication technology.
Thus, an optical spectrometer that is compact stable, robust, easy to assemble, and that offers high wavelength resolution is desirable. It is particularly desirable if the resolution allowed measurement of adjacent channels in a WDM system having a channel spacing of 200 GHz or less.
The invention provides a variable filter-based optical spectrometer using a Fabry-Perot (etalon) structure having high thermal stability in combination with a detector array. Short-pass, long-pass, or narrow band-pass filters can be used. The stability of the thin-film reflectors and intervening spacer region allows enhanced wavelength resolution from characterization of the filter-detector assembly and reconstruction techniques.
The input signal is typically carried on an optical fiber and lenses are used to expand and deliver the essentially point source of light from a fiber end into an optical beam that illuminates the variable filter. Other types of optical waveguides or systems could be used instead of an optic fiber. The expanded beam is only about 5-12 mm across, thus allowing a relatively small optical detector array. A small detector array is particularly desirable when using potentially expensive compound semiconductors and/or when a small footprint or size is desired. In one embodiment, a linear InGaAs diode array having 256 elements is used. Generally, the dimensions of the filter and the beam size are matched to the detector array to provide the complete spectral range available from the filter, but the filter and detector array do not have to match the beam size, and do not have to be of the same size.
In a further embodiment, signal-processing techniques are used to enhance the wavelength resolution of an optical spectrometer system using a reconstruction method. The variable filter/detector array assembly is characterized by providing a series of input signals of known wavelength and spectral profile, such as a series of laser inputs at wavelength intervals of 0.5 angstroms for a variable filter/diode array assembly having a nominal (as-measured) resolution of 8 angstroms or less. The accumulated response is used to create a transfer function that is applied to a subsequently measured optical signal to enhance the resolution of the optical spectrometer system, in some instances by a factor of 5 or more.